My intention in obtaining ttie K99/R00 award is to extend my research into studies of in vivo interplay between DNA repeat sequence instability, DNA damage and repair in the field of human disease-associated repeat sequence instability that is induced by environmental stress and mediated by base excision repair (BER). IVIy long-term goal is to understand how exposure to environmental stress influences the development and progression of human diseases through initiating and modulating repeat sequence instability and how the environmentallv-induced effects can be prevented by DNA damage repair. I hypothesize that environmental oxidative DNA damage and its inefficient BER is involved in CAG repeat expansion. The hypothesis will be explored by two Specific Aims. Aim one is to determine how inefficient processing of oxidative single-strand DNA (ssDNA) break intermediates induced by environmental carcinogens, chromate and bromate may be involved in CAG repeat expansion. The impact of insufficient processing of ssDNA breaks on CAG repeat expansion will be determined under deficiency of Pol p dRP lyase and FEN1 cleavage of hairpin structures. Aim two is to determine if highly efficient processing of oxidative ssDNA breaks can prevent CAG repeat expansion. This aim will be examined by determining if CAG repeat expansion can be reduced by efficient processing of ssDNA break intermediates through BER protein interactions between APE1 and Pol p, XRCC1 and Pol 3, as well as BLM and FEN1. Under Dr. Wilson's mentoring, I have successfully accomplished my research and career goals during the mentored phase of the award period. I have established several approaches for analyzing CAG repeat instability in vivo and in vitro. This has advanced my skills and knowledge in analyzing in vivo TNR stability and BER biochemistry. This also consolidated the basis for fulfillment of research goals during the independent phase as well as construction of my future R01 proposals. My future research will emphasize work in the areas of cellular and biochemical studies on repeat sequence instability induced by environmental oxidative DNA damage. BER mutational effects on trinucleotide repeat stability and BER protein biochemistry.